Extensive research has been done over the years in an effort to economically and reproducibly produce dense, sintered ceramic articles with a uniform microstructure starting with ceramic powders. Generally, in order to achieve high density, ceramic powders of selected particle size have been treated in various ways including, for example, pressure sintering at high temperature; the mixing of the powders with binders followed by low temperature heat treatment to form a `green` ceramic article which is then sintered at high temperatures; and the mixing of a flux (sintering aid) with the ceramic particles to reduce the sintering temperatures. To reproducibly manufacture reliable crystalline ceramic articles one must control the characteristics of the starting powder and the forming process parameters such that compacts of uniform high green density with interparticle pore size typically no larger than a single particle are achieved. For reproducible densification with control over the properties of the sintered ceramic one should also control the sintering variables such that pore removal occurs during sintering and grain boundaries develop between the particles.
During powder processing in liquid vehicles, one generally requires a stable particle dispersion, i.e. essentially free of agglomerates. It has been reported that the addition of surfactants to the liquid may be used to attain stability. Ellen S. Tormey, in "The Use of Surfactants in the Processing of High-Technology Electronic Ceramics" states "Achievement of a stable dispersion requires the formation of repulsive interparticle forces. In aqueous systems, electrostatic repulsion is generally dominant and arises due to the interactions between the electric double layers surrounding the dispersed particles. In nonpolar organic media (e.g., hydrocarbons) stability arises due to repulsion between interacting molecules adsorbed onto the particle surfaces and is generally referred to as steric stabilization. As a general rule, in the latter system, the most effective dispersants have strongly adsorbed functional groups and strongly solvated chains which extend into the solvent. Systems which are stabilized by a combination of mechanisms (i.e., charge and steric) tend to be the most stable. Steric stabilization can be effective in both aqueous and nonaqueous media, whereas the electrostatic mechanism is generally only effective in water or polar organic solvents. Most importantly, steric stabilization is effective in dispersions containing high volume fractions of solids, typically used to process ceramics, whereas electrostatic stabilization is generally only effective in dilute systems." She then teaches the use of such organic dispersants for use in particle size classification of ceramic powders. She further teaches "the dispersants typically used in ceramics processing bond to the particle surfaces via hydrogen bonding or weak chemical bonding". She recognizes that "dispersants which can chemically react (couple) with the particle surfaces to form stronger bonds offer distinct advantages in powder processing. Formation of a strong surface chemical bond would ensure that the dispersant remains on the particle surfaces during subsequent processing steps, resulting in a system which is less sensitive to slight compositional or processing condition variations. Coupling agent type dispersants would be especially advantageous in tape casting systems, since they are multicomponent and competitive adsorption is likely to be operative". Parish and Lalanandham have investigated the use of low molecular weight organotitanates as dispersants for ceramic powders (BaTiO.sub.3, Al.sub.2 O.sub.3) in nonpolar organic solvents such as hexane and toluene. When using coupling agents as dispersants for electronic ceramics, the metallic portion of the molecule must not be detrimental to the electrical properties of the ceramic or interfere with the sintering process, since this portion is not removed from the body during sintering. In the case of a metal-oxygen linked organic chain, the dispersant will decompose during sintering to a metal oxide residue.
Forney further teaches that in theory, coupling agents can serve not only as dispersants in ceramics powder processing, but also as dopants and/or binders. Dopants (generally secondary metallic oxides) are often added to ceramic powders to aid in the sintering process. For example, M.sub.g O is a well known sintering aid for Al.sub.2 O.sub.3 ; likewise, Y.sub.2 O.sub.3 is commonly used to enhance the densification of AlN. The addition of such a dopant in the form of a coupling agent would ensure that it is homogeneously distributed in the green body and also the sintered ceramic, since it bonds to the particle surfaces. For use as binders, coupling agents with polymerizable ligands can be synthesized. Such a molecule could first act as a dispersant and then be converted to a binder via an in situ polymerization step.
While much has been learned about the formation of high density sintered ceramic articles, the ability to economically and reproducibly produce such articles utilizing some ceramics, e.g. silicon nitride (Si.sub.3 N.sub.4), has evaded scientists. Silicon nitride has numerous desirable physical and chemical properties which, if it could be economically and reproducibly manufactured, especially in high density form, would make it particularly attractive in both wear resistant and high temperature applications. High density Si.sub.3 N.sub.4 ceramic articles have heretofore been difficult to form due to the largely covalent bonding and limited self diffusion, requiring not only high temperatures but pressure as high as 1.5 GPa. Dense Si.sub.3 N.sub.4 has been obtained by the use of powdered additives which, during the sintering process, provide a liquid phase to promote densification. However, the previously employed additives also introduce unwanted secondary phases which deteriorate the high-temperature mechanical properties of the densified material. Such prior art powdered additives have generally been employed in amounts ranging from 5-20+ weight % depending on the densification procedure, e.g. pressureless sintering, high pressure sintering, hot-pressing, and hot-isostatic pressing. The additives have most often been added by ball or attrition milling with the ceramic powder. This can adversely affect the characteristics and purity of the powder and distribution of the additives is not uniform. Another method reported in the literature for introducing the additive is the precipitation of the additive from a dispersion containing the Si.sub.3 N.sub.4 powder, however, while the additive is introduced somewhat more uniformly by this method, excessive shrinkage during sintering was encountered.
The method of the present invention incorporates sintering aid additives in a controlled, uniform, reproducible manner, providing a liquid phase at the sintering temperature which is in uniform and intimate contact with the ceramic powder grains, thus improving the kinetics for densification and transformation and allowing for the economical and reproducible formation of densified green or sintered ceramics, including silicon nitride, from powdered starting materials.